Although several growth factors which effect the differentiation of hematopoietic cells have been cloned, the physiologic role, if any, of these factors in hematopoietic stem cell self-renewal is unknown. Hematopoietic stem cells proliferate and differentiate both in vivo and in vitro in close association with a heterogeneous group of stromal cells, termed the hematopoietic microenvironment. Long term marrow cultures, a culture system which effectively mimics the hematopoietic microenvironment, stimulates the proliferation of stem cells in vitro for several months. The proliferation of stem cells in long term marrow cultures is dependent on direct stem cell stromal contact. Although long term marrow cultures are made up of multiple cell types, cloned cell lines have been established from such cultures which can effectively replace the complex cell mixture with a single stromal cell type. Stem cell-hematopoietic microenvironment interactions can be further simplified by the use of factor-dependent multipotent hematopoietic cell lines, such as FDCP cells. Direct contact with the hematopoietic microenvironment abrogates the factor-dependence of such cell lines even in the absence of measurable quantities of the growth factor in the hematopoietic microenvironment. Thus, evidence suggests that hematopoietic stem cell adhesion to the hematopoietic microenvironment may play a key role in control of stem cell proliferation, differentiation and, potentially in stem cell homing. We propose to study in detail the interaction of hematopoietic stem cells with the normal hematopoietic microenvironment and with the defective hematopoietic microenvironment present in the steel mouse mutant. A focus of this work will be the analysis of proteins secreted by such cloned stromal cell lines making up the extracellular matrix. These studies should provide new insight into the role of hematopoietic microenvironment extracellular matrix proteins in normal stem cell adhesion and in the abnormal hematopoiesis present in the steel mutation. These data will be useful in understanding changes in stem cell-microenvironment interaction which may accompany leukemic cell proliferation. In addition, the proposed studies will provide useful information for maintenance of optimal conditions for hematopoietic stem cells during in vitro manipulations required for retroviral mediated gene transfer, a requirement for future somatic gene therapy.